Improving aircraft composite inspections using optimized reference standards Page: 7 of 13
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when closer ply matches were used for calibration. As a
result, the prototype standard set was not altered and it
was concluded that 3, 6, 9, and 12 plies are needed to
set up NDT equipment for the expected range of
laminate thicknesses. Finally, NDI testing using bond
testers (high and low frequency), pulse-echo ultrasonics,
and machanical impedance analysis demonstrated the
difficulty of inspecting structures with 12 or more plies.
While acceptable SIN results could often be obtained,
the inspection results were not consistent.
REFERENCE STANDARD DESIGN &
FABRICATION - Further field testing was identified to
complete the validation of the prototype honeycomb
reference standard set (see "Future Activities" section
below). However, before proceeding with this final
phase of the validation, it was decided to reach some
conclusions on the standard fabrication process.
Several of the NDI tests highlighted some
inconsistencies in the flaw manufacturing methods.
Pillow insert flaws were used because it was thought
that they could provide realistic flaw responses.
However, it was determined that the response from the
disbonds and delaminations engineered with pillow
inserts sometimes did not provide a sufficient deviation
from the noise floor to allow for clear flaw detection.
Inspection results from the entire suite of specimens
generated thus far in the study proved that machining
the honeycomb core (recessing) away from the laminate
provides the best way of producing reliable skin-to-core
disbond flaws. This method also produces flaw sites
that can support tap testing. The remaining question is
how to realistically and repeatably produce interply
To answer this question, two trial standards
were manufactured that included three candidate
methods for engineering delamination flaws. Figure 5
shows the engineering drawing for these evaluation
honeycomb specimens; one carbon and one fiberglass
skin specimen was produced with this flaw layout. The
three methods employed to engineer the delamination
flaws were as follows: 1) pillow insert consisting of
Kapton tape around 4 layers of tissue paper, 2) brass
shims coated with a Silicon mold release to prevent
bonding to the plies, and 3) Teflon inserts. Each flaw
method was used to generate three like delamination
flaws in order to test for repeatability, as well as, to
statistically determine the amount of NDI signal
disruption generated by the flaw method. Note also that
the trial specimen includes potted core and core splice
areas. In order to expand the utilization of these
standards, potted core and core splice areas were
included as a tool to aid the interpretation of NDI signals.
This will help minimize false calls caused by the
presence of potted cores or core splices that will alter
NDT equipment readings.
* POTTED CORE
SKIN TO CORE DIS
* TEFLON INSERTS BRASS SHIMS
00 CORE SPLICE
ISBONDS PULL TAB
Figure 5: Engineering Drawing to Evaluate Honeycomb Reference
Standard Design and Fabrication
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Roach, D.; Dorrell, L.; Kollgaard, J. & Dreher, T. Improving aircraft composite inspections using optimized reference standards, article, October 1, 1998; United States. (https://digital.library.unt.edu/ark:/67531/metadc711418/m1/7/: accessed May 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.